Adds default options

I've added support for default options in the spec, such that:

   ...
   arg=5    -a=        Some argument
   ...
   opts, err := spec.Interpret([]string{"xyz"}, []string{})
   opts.GetInt("arg") == 5
   opts, err := spec.Interpret([]string{"xyz", "-a=2"}, []string{})
   opts.GetInt("arg") == 2